1. Field of the Invention
This invention relates to a vehicle navigation system for performing travel guidance with respect to an intersection when a vehicle approaches the intersection on a destination route set for navigating the vehicle to a destination.
2. Description of the Related Art
Vehicle navigation systems for navigating a vehicle to a destination smoothly by displaying a present position of the vehicle, which is detected by a GPA or the like, together with a road map are widely known. Further, vehicle navigation systems for navigating a vehicle to a destination by setting and utilizing an appropriate route (destination route) from a present position to the destination are widely known. These navigation systems contribute to smooth drives of the vehicles.
One type of these navigation systems issues vocal travel guidance for an intersection (guidance target intersection) to a user (driver) on the route every when a vehicle approaches the guidance target intersection. For instance, the navigation system issues vocal travel guidance for teaching a distance from the present position to the guidance target intersection and a direction to turn at the intersection in a manner such as that "Please travel straight .largecircle..largecircle. meters and turn right". Further, the navigation system can teach a name of the guidance target intersection in vocal sounds.
In this case, the timing (regular guidance timing) for performing the travel guidance is when the vehicle reaches a position having a specific distance from the guidance target intersection, and the specific distance is set in accordance with a type of road on which the vehicle travels. There is a case where vocal travel guidance is performed more than one time for one guidance target intersection. However, herebelow considered is a regular guidance timing (minimum regular guidance timing) for performing vocal travel guidance for the intersection latest. When a distance between two guidance target intersections is sufficiently long, the minimum regular guidance timing occurs inevitably. However, there is a case where the distance between two guidance target intersections is so short that the minimum regular guidance timing does not occur.
A conventional method to comply with such a case is explained below with reference to FIG. 9. It is assumed that a minimum regular guidance timing for a first guidance target intersection A is set to a time when a vehicle reaches a vehicle position P1 having a specific distance a0 from the first guidance target intersection A. In this case, usually, travel guidance is performed at the vehicle position P1 only for the first guidance target intersection A in a manner such as that "Please turn right". However, in a situation shown in FIG. 9, a distance L2 between the first guidance target intersection A and a second guidance target intersection B is shorter than the specific distance a0. The specific distance a0 also determines the minimum regular guidance timing for the intersection B. In this case, the travel guidance for the intersection B cannot be performed on the rout between the two intersections A and B.
Therefore, the travel guidance for the intersection B is performed together with the travel guidance for the intersection A at the vehicle position P1 in a manner such as that "Please turn right at the intersection A, and turn left at the next intersection B". When the situation where a distance between adjacent two guidance target intersections is shorter than the specific distance a0 continues from the first, second, third, . . . , to n guidance target intersections, the travel guidance may be performed for the first to n guidance target intersections simultaneously at the minimum regular guidance timing for the first guidance target intersection A.
For instance, when a road has two lanes after passing through the first guidance target intersection A as shown in FIG. 10, the user (driver) can make a decision by the travel guidance performed as above to enter the left side lane as indicated by line V1 immediately after passing through the intersection A or to change the lane from the right side to the left side as indicated by line V2. Accordingly, the vehicle can turn left at the intersection B smoothly.
However, the driver may miss hearing or forget the travel guidance for the second guidance target intersection B, because it is performed before passing through the first guidance target intersection A. If not only the distance L1 but also a distance L2 (see FIG. 10) between the second guidance target intersection B and a third guidance target intersection C is shorter than the distance a0, the travel guidance must be performed at the vehicle position P1 with respect to the first to third guidance target intersections A, B, and C. In this case, the driver is liable to miss hearing or forget the travel guidance more readily. To avoid this problem, it is important for the driver to hear the travel guidance immediately before passing through the intersection. In the conventional navigation system, there is a case where travel guidance is performed for three or more guidance target intersections simultaneously. In this case, the above problem becomes serious.